GB2190021A

GB2190021A - Temperature sensitive insulated conductors

Info

Publication number: GB2190021A
Application number: GB08709103A
Authority: GB
Inventors: Milton Sharples Greenhalgh
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1986-05-01
Filing date: 1987-04-15
Publication date: 1987-11-11
Also published as: DE3712771A1; FR2598251B1; GB8709103D0; GB2190021B; FR2598251A1; JPS62281303A; DE3712771B4

Description

GB 2 190 021 A 1

SPECIFICATION

Improved temperature sensitive solid 5 The present invention relates to improved electrical insulation materials. These materials have a particular thermal sensitivity characteristic that is usef u 1 in heat generating and heat detecting appa ratus.

Insu lating materia Is having tem perature-dependent electrical resistance or capacitance characteristics have long been extensively used in overheat sensing and control appi ications. Thus, by virtue of the inven tion of Spooner and Greenhalg h disclosed and claimed in U.S. Patent 2, 581,212 overheat protection for 10 electric blankets and si m i lar articles is provided for the use of such materials to afford the essential safety factor. In accordance with the teachings of that patent, the insulating material is operatively associated with switch means and is coextensive with the heating element so that when the tem pertu re anywhere in the blanket exceeds a predetermined maxim urn, the bla nket heating power su pply is interru pted. Because this insulating material is not altered physically or otherwise irreversiby changed in so functioning, it is useful 15 repeatedly for this purpose as it acts as a sort of electrical switch constantly monitoring the blanket operating temperature limit.

Avariety of insulating materials are identified in the prior art as being suitableforsuch use. Those include in addition tothe preferred Nylon polyamide resin of the aforesaid patent, polymeric organic materiaissuch as polyvinyl chloride and cellulose esters containing additives imparting the desired electrical char- 20 acteristics. In Japan and in the United Kingdom, compositions of polyvinyl chloride resin are used thatare "doped" with an ionizable quaternary ammonium compound. In U.S. Patent No. 2,745,944to Price,still another kind of material forthis same purpose, sulphur-cured butadiene- acrylonitrile elastomer is disclosed.

That material and all the others of the priorart, however, are in one respect or another, lessthan what has been desired and general recognition of thatfact hasfailed heretoforeto result in a thermal-sensing insulating material approaching the ideal which would combinethe best properties and characteristics of each of those, butwould befree, at leastto a large degree, from their major drawbackswhich are relatively low levels and ratios of changes in impedancewith temperature and, in the case of DCvolume resistivity, high levelsof volume resistivity and low ratios of changes in volume resistivityto temperature. In addition as in the case of Nylon resin,the effect of humidity shiftsthe levels of impedance resistivityto the extentthat control circuits 30 become a problem.

The practical significance of such shortcomings of prior art thermalsensing materials is apparent from the commercial electric blanket experience. Thus, when exposed to moisture, the Nylon insulation loses its desirable electric properties to a large extent in only an hour ortwo, and even though that insulation in an electric blanket is covered by a layer of polyethlene and an overlayer of polyvinyl chloride as described in the 35 Spooner and Greenhaigh patent reference above, moisture-induced deterioration occurs at a substantial rate. It is forthis reason that there have been operating difficulties with electric blankets in regions of high humidity such as the Texas Gulf Coast. The possibility of limiting the consequences of this destructive effect of high moisture-content atmospheres by increasing the shut-off temperature level so as to prolong the useful life of the Nylond insulation is not attractive forthe reason thatthe safety factor is thereby substantially 40 diminished.

The catastrophic effect of the presence of free sulfur in thermal-sensing insulation on wires of the type used in electric blanket structures has been demonstrated in tests under normal operating conditions running only six hours to wire failure.

On the basis of my discoveries and new concepts setforth below, the thermal-sensing insulating materials 45 previously known are improved and important new results can be obtained. Thus, by virtue of my invention, it is now possible to produce an insulated electrical conductorthat is more sensitive to changes in tem perature than prior conductors. The insulation of the invention is relatively insensitive to atmospheric moisture fluctuations and is resistant to stress cracking and overheat detection capability matching that of the best heretofore known. In fact, in preferred form, the thermal- sensing capability of the insulating com50 positions of this invention is substantially greaterthan any of the prior art, the specific inductive capacitance (S.I.C.) WC to room temperature ratio being higher by a factor of at least 3 to more than 5. Further, unlike some thermal-sensitive prior art conductor coverings, all these new insulating compositions are freef rom the tendencyto cause conductor corrosion and may be thermoset, they are not subject in use eitherto deformation orto additive migration detrimental to desired or intended purpose orfunction. Still another 55 important advantage of the insulating compositions of this invention overthose known heretofore isthat they are amenable to compounding with additives enhancing theirtemperature-sensitivity characteristics.

The composition comprises acrylonitrile-butadiene rubbers that contain substantially nofree orelemental sulphur, a polyvinyl chloride resin and a quaternary ammonium compound affording the advantages set forth above. 60 The proportion of acrylonitrile in these rubbers determines the extentto which they have the desired electrical response. Generally, they will comprise from 5 to 95 parts by weight and preferably 20to 45 partsof acrylonitrile based on the total weight of acrylonitrile and butadiene. In addition by using carboxylated acry lonitrile-butadiene asthe base elastomer in these compounds, their desirable characteristics can be substan tially improved without incurring any offsetting disadvantage or detrimental effect. In particular, when the 65 2 GB 2 190 021 A 2 compositions contain at least 0.5% of carboxylic units by weight, resistance to high temperature aging is enhanced as are physical properties together with the ratio of change in electrical response for both S.I.C.

and volume resistivity as the temperature is increased from room temperature to 900C.

Clay, and particularly kaolins such as Catalpo clay, can enhance the desirable electrical properties of these insulating materials. In the case of Catalpo (Freeport Kaolin Co. trademark) clay, for instance, both the S.I.C. 5 ratio and the volume resistance are altered to great advantage through its use as will be shown in detail below. This technology of selective compounding can be extended to otherfillers such as silicas, carbon black, etc., as well as plasticizers, in accordance with my furtherfindings.

The compound or composition of the invention wil I comprise from 10 to 95 parts by weight and preferably from 40 to 60 parts by weight of acrylonitrile-butatiene rubber based on the total weight of composition as 10 parts; from 5 to 95 by weight and preferably from 40 to 60 parts by weight of polyvinyl chloride resin based on the total weight of composition as 100 parts; from 0.1 to 1.0 mole and preferably from 0.25 to 0.75 mole of a quaternary ammonium compound per liter of compound. In addition, if desired, the composition may contain from 5 to 95 parts by weight, and preferably from 10 to 80 parts by weight of clay based on the total weight of composition as 100 parts. Additives, such as pigments, stabilizers, curing agents, flame re- 15 tardants and the like may be employed in conventional quantitives.

A continuous thermo-sensitive construction designed forthe electrical blanket must be extremely flexible, have no odor, be non-staining to fabrics, have certain heat aging requirements, and capable of withstanding any cleansing operation. On the other hand, for hotspot detection such as in a fire alarm system, the corn pounded material need not beas flexible, non-odorous, but should be tougher so that it can be readily 20 installed in various location.

The quaternary ammonium compound of the invention may be of theformula R' + 25 R 2 -N-R 4 X R 3 wherein R'; R 2; R'and R'are independently selected from the group consisting of alkyl of 1 to 20carbons, 30 alkenyl of 1 to 20 carbon atoms, phenyl, alkylphenyl wherein the alkyl group has 1 to 10 carbons; phenylalkyl wherein the alkyl group has from 1 to 10 carbon atoms; X is a halogen, preferably chlorine. The preferred quaternary ammonium compounds are cetyl trimethyl ammonium and stearyl dimethyle benzy] ammonium chloride.

The polymeric material selected should contain substantially no free sulfur. In the case of the acrylonitrile- 35 butadiene formulations, the acrylonitrile must be present in amount of at least 1 %; and forthe carboxylated material, the acid monomer units should be present in amount of at leastO. 5%.

In addition to being curable by either sulfur-bearing combinations or peroxide, the carboxylated polymer can be cured by zinc oxide. Further, both polymers (polyboxylated and non- carboxylated acrylonitrilebutadiene) can be used. 40 In its method aspect,this invention comprisesthe steps of contacting and thereby covering at leasta portion of the length of metal wirewith the thermal -sensitive polymeric material of the invention.

Similarly, in its article-of-manufacture aspect,this invention comprises at leasta portion of an electrically conductive membersuch as length of wire, and a covering on and in contact directly with the conductive member orwire,the covering comprising a thermal-sensitive polymeric material as defined above. 45 The conductor may be used in an electric blanket but itwill be understoodthat it is useful in otherways and combination such as in association with smoke detector apparatus. A hotspot detector comprising a refer ence conductorand a sensing conductorwhich are connectableto an electric powersource, overheatsignal oralarm means, and actuating means operatively associated with the signal means and thetwo conductors to actuatethe signal meanswhen D.C. resistance between the conductors exceeds a predetermined max- 50 imum. The conductors are wires spread uniformly apart overtheirfull lengths and encased in a bodyof sensitive polymeric material which fills the space of 10 mils or so between thetwo conductors and which isof the novel composition and unique properties described above. U.S. application Serial No. 548,376,filed November3,1981 described specific apparatus in which the improved insulation of this invention may be utilized. This application is incorporated herein by reference. 55 3 GB 2 190 021 A 3 Brief Description of the Drawings

Those skilled in the artwill gain a further and better understanding of the present invention in all its aspects upon consideration of the detailed description setforth below, taken in conjunction with the drawings ac companying and forming a part of this specification, in which:

Figure 1 is a graph of the relation of temperature to impedance for a composition of acrylonitrile-butadiene 5 and polyvinyl chloride resins.

Figure2 is a graph of the relation of temperatureto impedancefor a composition of acrylonitrile butadiene, polyvinyl chloride resin and stearyl dimethyl benzyl ammonium chloride.

Figure3 is a graph of the relation of temperatureto impedancefor a composition of acrylonitrile- butadiene, polyvinyl chloride resin and cetyl trimethyl ammonium chloride. 10 Detailed Description of the preferredEmbodiments

Example 1

Acomposition prepared from the following ingredients: 15 Acrylonitrile-butadienel PVC (50:50 blend) - 100parts (Hycar 503 H) Tribasic lead sulfate (Tribase EXL) - 4 parts 20 Antioxidant (Stangard 500) - 2 parts Titanium dioxide - 2 parts Stearyl dimethyl benzy] ammonium chloride -.025 moles 25 108.25 parts The composition is prepared by means of a two roll rubber mill with the rollers set at about 150'F.The milled composition is sheeted off the mill and extruded into wire as an insulating layer.

30 Example2

The composition is prepared from thefollowing ingredients:

Acrylonitrile-butadiene/ PVC (50:50 blend) - 100parts (Hycar 503 F1) 35 Tribasic lead suffate (Tribase EXL) - 4 parts Antioxidant (Stangard 500) 2 parts Titanium dioxide - 2 parts 40 Cetyl trimethyl ammonium chloride -.025 moles 108.25 parts The composition is prepared by means of a two roll rubber mill with the rollers set at about 150'F.The 45 milled composition is sheeted off the mill and extruded onto wire as an insulating layer.

The acrylonitrile appearsto function as a non-migratory plasticizer which aids in retaining the quaternary salt in the composition.

CLAWS 50 1. A method of producing an insulated electrical conductor having special utility in overheat detection applications, which comprises the step of contacting and thereby covering at least a portion of the length of a metal wire with thermal-sensitive polymeric material which is relatively insensitive to atmospheric moisture fluctuations and is resistant to stress cracking and to corrosion, said polymeric material being selected from 55 the group consisting of (1) a thermosetting acylonitrile-butadiene rubber containing at least one percent by weight or acrylonitrile and polyvinyl chloride resin, (2) thermoplastic blends of a polyvinyl chloride and an acrylonitrilebutadiene rubber containing at least one percent of acrylonitrile, in which thermosetting rubber (1) after curing and thermoplastic blend (2) contain substantially no free sulphur, including adding an amount of a quaternary ammonium compound to said polymeric material to increase the sensitivity of 60 thermally sensitive polymeric material to changes in impedance.

Claims

2. The method of Claim 1, in which the ratio of specific inductive

capacitance of the acrylonitrile butadiene rubberfrom 900C to room temperature is greater than about 10.

3. The method of Claim 1 in which the acrylonitrile-butadiene rubber contains 20% to 45% by weight of acrylonitrile monomer units. 65 4 GB 2 190 021 A 4 4. The method of Claim 6 in which the thermosetting carboxylated acrylonitrile-butadiene rubber of (1) is cured with a metal oxide.

5. The method of Claim 1 in which the acrylonitrile-butadiene rubber of (1) or (2) contains at least 0.5 percent by weight of carboxylic acid monomer units.

6. The method of Claim 1 in which the polymeric material is a thermoplastic blend of an acrylonitrile- 5 butadient rubber containing at least 5% of carboxylic acid monomer units and from 5% to 95% of polyvinyl chloride.

7. An electrically-insulated article of manufacture comprising:

(a) at least a portion of an electrically conductive member; and (b) a covering on and in contactwith said portion of the conducting member, said covering comprising a 10 thermal-sensitive polymeric material which is relatively insensitiveto atmospheric moisture fluctuations and is resistant to stress cracking and to corrosion, said polymeric material being selected from the group con sisting of (1) a thermosetting acrylonitrile-butadiene rubber containing at least one percent byweight of acrylonitrile and polyvinyl chloride, (2) thermoplastic blends of a polyvinyl chloride and an acrylonitrile- butadiene rubber containing at least one percent of acrylonitrile, in which thermosetting rubber (1) after 15 curing and termoplastic blend (2) contain substantially no free sulphur, said covering including an amount of a quaternary ammonium compound to increasethe sensitivity of the polymeric material to changes in impe dance.

8. The method of Claim 5 in which the acrylonitrile-butadiene rubber contains two to six percent by weight of carboxylic acid monomer units. 20 9. A method as claimed in claim 1, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

10. A method as claimed in claim 1, substantially as hereinbefore described in anyone of the examples.

11. An electrically insulated article as claimed in claim 7, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings. 25 12. An electrically insulated article as claimed in claim 7, substantially as hereinbefore described in any one of the examples.

13. An electrically insulated article when produced by a method as claimed in anyone of claims 1 to 6 and 8 to 10.

Printed for Her Majesty's Stationery Office by Croydon Printing Company (UK) Ltd,9187, D8991685.

Published byThe Patent Office, 25 Southampton Buildings, LondonWC2A lAY, from which copies maybe obtained.

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